Bone-supporting components such as bone plates and intramedullary nails (bone nails) have become a treatment of choice for the fixation of bone fractures, especially fractures of long bones (e.g., the humerus, tibia and femur), and in non-broken bones to prevent fractures. Typically, bone nails are rod-shaped devices configured and constructed to be secured (interlocked) to a bone using one or more fixation components which anchor the bone nail into the bone in order to carry the loads until the bone fracture is cured.
Bone plates are generally (but not exclusively) used in cases that a bone nail can't be used, and are designed for implantation on the bone surface.
Implant fixation components include bone screws, rods and pegs. Bone screws are used for fixation at one or both ends of a nail or along a bone plate Implant fixation components generally referred to as “pegs” are round unthreaded rods (or threaded at one end), conventionally formed of metal that are usually (but not exclusively) used to help anchor bone plates to a bone such as the proximal humerus or the distal radius for fracture fixation. The rod goes into a hole drilled into the bone.
In the art, the entire implant is generally constructed from metal, such as titanium, stainless steel, or a cobalt-chromium alloy. Although metallic implants provide numerous advantages, they also have a few drawbacks. Metal construction normally provides adequate bending strength, thus reducing problems associated with implant fracture. However, the rigid metal implant creates a relative high degree of stresses in certain regions of the bone, while, on the other hand, does not provide for sufficient load transfer resulting in stress shielding. Both high stress and stress shielding can cause bone deterioration and resorption, leading to areas of bone weakness and loss of bone support for the implant (e.g., intramedullary nails and stem components of joint replacement systems). In addition, metals may result in artifacts in CT and MR imaging. In addition metals can mask (i.e., block) radiation treatment in cancer cases. Furthermore, metals such as stainless steel and cobalt chromium may cause biocompatibility problems related to corrosion and sensitization reaction (mainly due to allergy to nickel).
Also, conventional metal fixation components, e.g., bone-supporting components (e.g., intramedullary nails and bone plates) and pegs and screws can mask the fracture and limit the ability of the surgeon to set the fracture correctly. Metal bone-supporting components and fixation components may also limit the ability to see the healing process in X-Rays.
Non-metal implants made of a lighter and more flexible material, yet having sufficient strength for load bearing, have been suggested in the past. In particular, composite material implants, for example formed of polymer reinforced with fibers, are disclosed in U.S. Pat. Nos. 4,750,905, 5,181,930, 5,397,358, 5,009,664, 5,064,439, 4,978,360 and 7,419,714, the disclosures of which are incorporated herein by reference.
U.S. Pat. No. 5,009,664 describes a tubular, curved marrow nail, made of carbon fibers, which are preferably knit in a crisscross fashion, saturated in a hardenable plastic, with a conically tapered distal tip.
U.S. Pat. No. 5,181,930 describes an implant comprising an elongated core formed of continuous filament fibers embedded in thermoplastic polymer. The core is encased within a filler, made of a non-reinforced polymer which is molded around the core to proximate the final desired shape of the implant. A sheath, composed of reinforced fibers embedded in a polymer, is spiral wound around the filler, at angles (orientations) which may vary along the implant axis.
Although known composite material implant technology can provide several advantages, there are also some limitations. In contrast to metal, composite material implants are radiolucent, i.e., do not block most of the radiation coming from x-ray systems such as fluoroscopy, and hence their implantation and tracking during follow-up may be difficult. For bone nails or plates, accurate insertion of the screws into the holes in the nail/plate is crucial to the success of the operation, especially where no aiming device is used.
U.S. Pat. No. 7,419,714 describes a bone screw or plate formed of a composite of polymer or ceramic material.
Currently available bone implants, such as bone plates, include pre-drilled holes for the fixation devices which anchor the implant to the bone and optionally lock the implant in place and/or provide for bone fragments compression.